The present invention relates to the selective hydrogenation of nitriles such as adiponitrile to omega-aminonitriles such as eta-aminocapronitrile, employing a Group VIII metal-containing catalyst.
Group VIII noble metals are well known catalysts for various hydrogenations of organic compounds. The complete hydrogenation of adiponitrile to hexamethylenediamine is normally conducted with an iron, nickel and/or cobalt catalyst; however, platinum or palladium catalysts have also been suggested for this reaction. While the diamine product is widely used for producing diacid-diamine-type polyamides such Nylon-66, it would be desirable to also produce from the dinitrile the partial hydrogenation product, eta-aminocapronitrile, since this compound could be cyclized into caprolactam and polymerized to produce Nylon-6. While some attempts have been made with certain Group VIII metals or complexes to achieve selective hydrogenation, a process with high selectivity to eta-aminocapronitrile, or other omega-aminonitriles, at moderate conversion and with a catalyst which can be easily handled and conveniently recycled have not yet been found.
Rhodium has found few practical applications in hydrogenations. Rhodium metal itself, and inorganic rhodium oxides and salts supported on an inert support have been used in certain applications. Examples of processes employing such inert rhodium materials are contained in P. N. Rylander, Catalytic Hydrogenation in Organic Synthesis (New York 1979). Rhodium-based catalysts have not, however, been known to selectively hydrogenate dinitriles to aminonitriles.
Y. Takagi et al., Scientific Papers institute Physical & Chemical Research (Japan), vol. 61, No. 3, pp. 114-17 (1967) discloses processes for hydrogenating nitriles with unsupported rhodium catalysts. In particular, rhodium hydroxide prepared by adding various amounts of sodium hydroxide to a hot aqueous solution of rhodium chloride was used as a catalyst for the hydrogenation of adiponitrile. Table II of the reference indicates that increased amounts of sodium hydroxide as a further additive increased the yield of hexamethylenediamine, but eventually retard the reaction rate. Lithium hydroxide was a superior additive. The yields were in most cases lower than the 80% yield reported using rhodium oxide prepared by the fusion of rhodium chloride with sodium nitrate.
M. Freifelder et al., J. Am. Chem. Soc., vol. 82, pp. 2386-2389 (1960) employed 5% rhodium on alumina to hydrogenate aliphatic nitriles and especially 3-indoleacetonitrile. Ammonia was present to suppress production of secondary amines, but also reduced the catalytic activity of the rhodium. Ammonia is known to suppress secondary amine formation and reduce activity with other Group VIII metal catalysts.
U.S. Pat. Nos. 2,208,598 and 2,257,814, each to Rigby (Dupont 1940), and German Pat. Nos. 836,938 (1952), 848,654 (1952) and 954,416 (1956), all to BASF, disclose various catalytic processes directed to producing omega-aminonitriles from dinitriles employing miscellaneous catalysts including Raney nickel and iron, but not specifically any of the platinum group. See also U.S. Pat. No. 2,762,835 to Swerdloff (Celanese 1956).
Italian Pat. No. 845,999 to Montecatini Edison S.p.A. (1969) discloses the hydrogenation of dinitriles one or two carbons shorter than adiponitrile (e.g. succinonitrile) in the presence of a rhodium catalyst and ammonia to produce an omega-aminonitrile.
None of these patents provides a process with high selectivity to aminonitrile (compared to diamine and secondary amine) at high conversions of dinitrile with high rates of catalyst turnover and long catalyst life.